This invention relates to an image processing device having an optical scanning system, and more particularly to an image forming device provided with an optical scanning system which deflects a laser beam in a constant main direction on a recording medium such as a photosensitive drum.
A laser beam printer is representative of image forming devices of this type. A laser beam printer has a laser diode and a rotating polygonal mirror. A laser beam emitted from a laser diode is deflected by the polygon mirror so as to scan a circumferential surface of a photosensitive medium in a drum shape in a main scanning direction. In other words, the laser beam from the polygon mirror is continuously irradiated on the surface of the photosensitive drum in a direction parallel to the drum axis. The above-mentioned scan is repeatedly carried out. (This process is called line scanning). During each of these line scans, the laser beam is turned ON or OFF for every one dot. Generally, the laser beam is turned ON when one dot is to be recorded On the other hand, the laser beam is turned OFF when no recording is indicated. The surface of the photosensitive drum is uniformly charged prior to being exposed by the laser light beam and the charge is removed from the exposed parts so that an electrostatic latent image is formed thereon. Toner is then applied to this electrostatic latent image and developing is performed so that a toner image is produced. Thereafter, the toner image is transferred to transfer paper.
Various types of laser scanning means for performing the main scanning of the photosensitive drum have been proposed. For example, there are those that use a Galvano mirror, those that use a curved polygonal mirror, those that use a rotating polygonal mirror (which is a combination of a rotating polygonal mirror and a deflector), each in combination with a flat lens, or a rotating deflector and an F.theta. lens, and the like.
In all cases, the beam of the laser diode moves back and forth at an equivalent angular velocity, in the main scanning direction. However, the photosensitive drum that receives this light at the center of its motion, has a linear shape and so the line scanning speed on the photosensitive drum becomes unequal for within the same line. For this reason, the exposure time (scanning time) for an area of one dot on any one line is long for the points close to the midpoint, and shorter for the further a point is distant from the midpoint. Because of this, many equal line scanning speed means have been proposed, amongst them being the use of polygonal mirrors with curved surfaces, and the use of F.theta. lenses.
However, these mechanical and optical means for achieving an equal scanning speed must be precise and their manufacture and processing is not simple. The reason for this is that a rational means must be found to produce equal speed compensation for across an entire scanning line which is relatively long when compared to the series of extremely small virtual dots that include it.
Therefore, it is desirable that these equal speed means be either eliminated altogether or be substituted by some means by which a relatively rough level of compensation can be performed. In order to do this, for example, it is possible in the image write processing circuit that forms the image in dot units, to give a main scanning synchronization signal that renews the image signal in dot units for the middle portion of the scan line, and to make the cycle shorter for the further a point is away from the middle portion. (This is known as frequency modulation.) Moreover, it is also possible to reduce the power to the laser diode for dots in the middle portion and to make the cycle longer for the further a point is away from the middle portion. (This is known as power modulation.) Japanese Patent Laid Open Application No. 76572/1988 is one such image forming device that has been proposed.
With such devices, then the above-mentioned frequency modulation and power modulation make the amount of light received for each dot in the said exposure line effectively the same even if the scanning of the exposure line on the photosensitive drum is performed at an unequal angular speed because of the beam moving back and forth at a constant angular speed. Moreover, the scanning of each dot and the image signals allocated to that dot can be synchronized so that there is no effective change in either the shape or the recording density of the image. Accordingly, mechanical and optical equal speed means can be eliminated and a relatively rough level of compensation can be performed for the equal speed means.
In laser printers of this type, the main unit of the laser beam printer is generally common and the previously described scanning means are made as units. It is desirable that only these scanning means can be capable of being incorporated into a laser beam printer main unit in accordance with the type of machine required as this would facilitate parts management, assembly management, product adjustment, quality control, product management and other work.
However, as has been described above, there are various types of optical scanning means available and even those that use the same method have large differences in the manufacture and the production because of the differences in the elements configuring them.
Because of this, the main scanning characteristics of scanning means differ according to the optical scanning method used. Therefore, when an optical scanning unit is to be selectively incorporated into a laser beam printer main unit, and when a unit is to be exchanged for a new unit, it is necessary to have selective incorporation, renewal or replacement along with the electrical circuit elements that perform the frequency modulation and power modulation.
Therefore, the selective incorporation, renewal or replacement of the aforementioned electrical circuit elements accompanying the selective incorporation, renewal or replacement of the optical scanning unit has not been easy.